There are nearly 500 million Spanish/Castilian-speaking people in the world and a huge percentage of them -300 million, in fact- reside in Latin America.
Latin American Spanish contains a huge array of dialects, including Amazonian Spanish, Andean Spanish, Bolivian Spanish, Buenos Aires Spanish/River Plate Spanish, Caribbean Spanish, Central American Spanish, Chilean Spanish, Colombian-Ecuadorian Spanish, Paraguayan Spanish, Peruvian Spanish and Puerto Rican Spanish.
All of these dialects have unique idioms, accents and vocabulary. For example, there are two completely different words used in Castilian Spanish and Latin American Spanish to refer to a computer -computadora and ordenador. Each of these words sounds incorrect when used in the wrong region. Differences like this make it extremely important to write for the intended target when translating from any language into any form of Spanish to ensure that your target translation sounds both correct and natural.
With 109 million inhabitants, Mexico is the country with the largest Spanish-speaking population in the world. Mexican Spanish is based on the Aztec language, Nahuatl, with a large influence of Castilian Spanish. Castilian Spanish reach is confined to vocabulary, however, and there are great differences in the grammar of Castilian Spanish and Mexican Spanish. As with any country of Mexico's size, there are many dialects within Mexican Spanish. These dialects are influenced by the region, education and the socio-economic level.